Lightguide fiber is drawn from a solid glass cylinder or preform. One particular technique for fabricating such a preform is described in U.S. Pat. No. 4,062,665 to Izawa et al. which issued Dec. 13, 1977 and has been commonly referred to as the vapor-phase axial deposition (VAD) method.
The VAD technique uses a refractory starting member projecting axially into the central portion of an elongated cylindrical chamber. The member rotates along its axis of rotation, which is coincident with the chamber axis, as it is withdrawn therefrom. A glass raw material is introduced into a high temperature portion of a flame near the tip of a stationary torch producing a glassy particulate or soot which is directed onto the end of the rotating refractory starting member. A substantially cylindrical boule of porous soot is formed on the starting member as it is continuously withdrawn from the chamber at a rate equal to the growth rate of the soot upon the boule. The withdrawn soot boule is then subjected to an elevated temperature to consolidate the porous material into a solid, clear cylindrical lightguide preform. Such a technique has been found capable of fabricating lightguide preforms from which low loss lightguide fiber has been drawn.
It has been observed that extremely small variations in pressure (&lt;&lt;0.25 mm H.sub.2 O) within the VAD chamber will cause the flame to wander and flicker resulting in inefficient, nonuniform deposition, loss of reactant materials and a lack of repeatability between runs. Prior art VAD chambers have been substantially spherical or elongated cylinders. In such chambers large dead spaces are present which permit gas circulation due to the natural convection driven by the temperature difference between the torch-boule region and the chamber walls. Such secondary circulation is never symmetrical or regular and therefore, undesirably, induces random variations of the local flow and pressure fields at the flame-deposition site on the boule.
Accordingly, there is a need for a system for repeatably reproducing uniform, high quality, lightguide preforms using the VAD process.